1. Field of the Invention
The present invention relates to a silicon nitride sintered body having a high heat conductivity and to a structural element using a silicon nitride sintered body such as an insulating substrate used for forming a semiconductor device and various radiators.
2. Description of the Related Art
A ceramic sintered body containing silicon nitride as a main component can exhibit excellent resistance to heat and excellent resistance to thermal shock even under a high temperature environment not lower than 800° C. Therefore, it is attempted to use the ceramic sintered body noted above for forming various parts having a high strength and a high heat-resistant as a high temperature structural element. The ceramic sintered body containing silicon nitride as a main component also exhibits high corrosion resistance relative to a metal and, thus, it is attempted to use the sintered body noted above as a melting-resistant material of a vessel handling a molten metal. Further, the ceramic sintered body in question exhibits high resistance to abrasion and, thus, it is attempted to use the sintered body in a sliding member such as a bearing and in a cutting tool.
The known silicon nitride ceramic sintered body is prepared by, for example, adding a sintering agent such as an oxide of a rare earth element, e.g., yttrium oxide, or aluminum oxide to silicon nitride. In the conventional silicon nitride ceramic sintered body, the sintering properties are enhanced by the sintering agent noted above so as to densify and strengthen the sintered body.
Also, the conventional silicon nitride ceramic sintered body is manufactured by adding the sintering agent noted above to the silicon nitride powder, followed by molding the resultant mixture and then sintering the molded material at temperatures of about 1,700 to 1,850° C. for a prescribed time and subsequently cooling the sintered body in a furnace.
However, the silicon nitride ceramic sintered body manufactured by the conventional method described above, which was certainly excellent in mechanical strength, heat resistance and oxidation resistance, was not satisfactory in heat conductivity. Under the circumstances, it was highly required to develop a silicon nitride ceramic sintered body, which exhibits high heat conductivity as well as high mechanical strength, high heat resistance and high oxidation resistance.
In responce to the requirement noted above, the present inventors conducted extensive research on the raw materials, compositions and sintering methods of silicon nitride materials so as to propose previously a silicon nitride sintered body that exhibits a marked improvement in heat conductivity and a method of manufacturing the same in, for example, Japanese Patent Disclosure (Kokai) No. 7-48174, Japanese Patent Disclosure No. 6-135771 and Japanese Patent Disclosure No. 2000-34172 so as to arrive at the practical use of the silicon nitride sintered body as a substrate for a semiconductor device.
In the technology proposed in each of the three patent documents quoted above, the silicon nitride sintered body is allowed to exhibit high heat conductivity by decreasing the content of the impurity cationic element in the silicon nitride sintered body.
To be more specific, in the silicon nitride sintered body proposed in Japanese Patent Disclosure No. 7-48174 and Japanese Patent Disclosure No. 6-135771, a high heat conductivity not lower than 60 W/m·K is achieved by lowering the content of the sum of the impurity cationic elements of Fe, Mg, Ca, Sr, Ba, Mn, B, Li, Na and K to 0.3% by weight or less.
In contrast, in the silicon nitride sintered body disclosed in Japanese Patent Disclosure No. 2000-34172, a high heat conductivity not lower than 70 W/m·K, is achieved by lowering the content of the sum of the impurity cationic elements of Al, Li, Na, K, Fe, Ba, Mn and B to 0.3% by weight or less.
However, the silicon nitride sintered body disclosed in the three patent documents quoted above is not satisfactory in oxidation resistance at temperatures falling within a range of 800° C. to 1,000° C. so as to make it necessary to further improve the silicon nitride sintered body in order to use the silicon nitride sintered body as a material of a structural element having a corrosion resistant at high temperature. In addition, the silicon nitride sintered body disclosed in the patent documents quoted above leaves room for further improvement in terms of the manufacturing cost because, in order to decrease the content of the impurity cationic element in the silicon nitride sintered body, it is necessary to use a fine silicon nitride powder of a high purity.